SR: Gamma rays creating election-positron pair

[villiami]

Homework Statement

This is a special relativity question: Two gamma rays of identical energies E collide with an angle \theta between their velocities. Show that positron-electron pair creation is impossible in the limiting case \theta=0.

Find the minimum \theta value for p-e creation to the possible.

The Attempt at a Solution

Stuck! If E is greater than election rest mass, can't it happen at any angle?

 

[villiami]

Is it possibly that: with photons, there are no frames in which momentum=0.
If the p-e pair were produced, there will then be a frame in which momentum=0 (which previously had non-zero momentum photons).
Thus mom is not conserved?